It is possible with current systems to transmit films and audiovisual programs in digital form via broadcasting networks of the microwave cable, satellite type, etc. or via telecommunication networks of the DSL (digital subscriber line) type or BLR (local radio loop) type or via DAB (digital audio broadcasting) networks, etc. Moreover, to avoid the pirating of works broadcast in this manner, the latter are frequently encrypted or scrambled by various well known means.
Concerning the processing of video sequences encoded with wavelet technology, U.S. Pat. No. 6,370,197 discloses a method of coding a video sequence based on a wavelet transform and generating a nested digital stream. It does not propose any method for protecting the stream and/or scrambling the video sequence.
EP 0,734,164 presents a process and device for increasing the efficacy of coding brought about by video encoders based on the classified vectorial quantification by optimizing the coding in such a manner as to not have to transmit the classification information in the encoded binary stream. It applies to video streams stemming from a DCT transform or a wavelet transform. To this end, the entering video signal is divided into a plurality of subbands, e.g., the DC coefficients are arranged in one subband and the AC coefficients in remaining subbands, followed by a formatting in blocks of identical size, each block of which includes a DC coefficient and a multitude of AC coefficients. A selection signal is then generated, representing the vectorial quantification class corresponding to each assembled block. This stage is followed by classification for the vectorial quantification by generation of parameters relative to the evolution of the DC coefficients in the horizontal and vertical directions and by differential entropic encoding of the DC coefficients relative to the assembled blocks for generating a first encoded video signal. The AC coefficients are classified and encoded separately with the aid of an entropic encoding as a function of the selection information for generating a second encoded video signal. The two signals generated in this manner are formatted for the transmission.
For decoding, no classification information is transmitted. It is reconstructed from the DC coefficients encoded and transmitted to the decoder. That system concerns a process relative to the digital compression and the encoding of video streams stemming from the DCT transform and the wavelet transform. The description of the process indicates the stages to be applied for implementing a classified vectorial quantification that increases the compression and the effectiveness of the encoding. A single stream is transmitted to the receiver. The technical problem and the objective are to optimize the digital format and obtain a digital stream formatted at the output of a digital encoder. The process described in that system does not permit securing the video stream and does not offer protection against illicit uses of video streams stemming from encoding in wavelets.
As concerns the protection of images coded in wavelets, EP 1 033 880, discloses a process and device for protection by modifications applied to the spatial-frequency coefficients. These modifications are of the type: Modification of the sign bits of the coefficients, modification of the improvement bits of the coefficients, the choice of the appropriate coefficients belonging to a frequency subband for shifting (exchanging) them, rotation of a block regrouping the frequency coefficients arranged in increasing order while attempting to respect to the maximum the static properties and entropy of the original signal. Each type of modification is conditioned with the aid of a key. The data protected in this manner is then passed through an entropic encoder and a bitstream in conformity with the norm is generated. It encrypts with the aid of keys and, as a consequence a single stream is transmitted to the receiver and all the elements constituting the original stream are located within the protected stream. It does not respond in a satisfactory manner to the protection of the transmitted video stream. Moreover, as a consequence of the modifications before the entropic encoding the statistical properties are modified and the size of the stream and the transmission rate increase. Consequently, it does not satisfy the objectives of high security that guarantee a process without loss, the subject matter of the present invention.
WO 00/31964 A discloses a method and equipment for the partial encrypting of images to protect them and optimize the storage location. A first part of the image is compressed to a low quality without encryption and a second part of the image is encrypted. When the first and second part are reunited the image is obtained with maximal quality. The second part is encrypted and comprises two sections encrypted in different manners. The decryption of the first section and its combination with the first part restores the initial image with an average quality. The decryption of the second section, its combination with the first section and the first part restores the original image with maximal quality.
The image can also be partitioned into multiple independent sections, each section of which is encrypted with its own method and its own key. The protection method is the encryption and consequently all the original elements of the stream remain within the protected stream and restoration of the entire content from only the protected stream is possible in the instance that an ill-disposed person finds or simulates the encryption keys. It does not furnish satisfactory security against pirating of the video stream. Also, the size of the protected stream is different from the size of the original stream. It therefore does not resolve the problem of high security while procuring a fine granularity in the quality of the reconstituted video sequences processed in the present invention. U.S. Pat No. 2001/0053222proposes a process and system for the protection of video streams encoded according to the MPEG-4 norm. The audiovisual stream is composed of several audio and video objects managed by a scenic composition. One of the objects of the video stream is encrypted with the aid of a key generated in four encryption stages that is periodically renewed. The protected objects are video objects. The encrypted object is multiplexed with the other objects and the entire stream is sent to the user. The MPEG-4 stream is recomposed in the addressed equipment by the decryption module, that reconstitutes the original video stream from the encrypted video stream and by regenerating the encryption key from encryption information previously sent and from information contained in the encrypted stream. Given that the entire protected content of the video objects is located in the stream sent to the user, an ill-intentioned person who finds the encryption keys would be able to decrypt this protected content and to view or broadcast it. This prior art therefore does not entirely resolve the problem of securing the video stream.